dm: set DM_TARGET_WILDCARD feature on "error" target

The DM_TARGET_WILDCARD feature indicates that the "error" target may
replace any target; even immutable targets. This feature will be useful
to preserve the ability to replace the "multipath" target even once it
is formally converted over to having the DM_TARGET_IMMUTABLE feature.

Also, implicit in the DM_TARGET_WILDCARD feature flag being set is that
.map, .map_rq, .clone_and_map_rq and .release_clone_rq are all defined
in the target_type.

Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-ioctl.c

···
 
 	immutable_target_type = dm_get_immutable_target_type(md);
 	if (immutable_target_type &&
-	    (immutable_target_type != dm_table_get_immutable_target_type(t))) {
+	    (immutable_target_type != dm_table_get_immutable_target_type(t)) &&
+	    !dm_table_get_wildcard_target(t)) {
 		DMWARN("can't replace immutable target type %s",
 		       immutable_target_type->name);
 		r = -EINVAL;

drivers/md/dm-table.c

···
 	return t->immutable_target_type;
 }
 
+struct dm_target *dm_table_get_wildcard_target(struct dm_table *t)
+{
+	struct dm_target *uninitialized_var(ti);
+	unsigned i = 0;
+
+	while (i < dm_table_get_num_targets(t)) {
+		ti = dm_table_get_target(t, i++);
+		if (dm_target_is_wildcard(ti->type))
+			return ti;
+	}
+
+	return NULL;
+}
+
 bool dm_table_request_based(struct dm_table *t)
 {
 	return __table_type_request_based(dm_table_get_type(t));

drivers/md/dm-target.c

···
 
 static struct target_type error_target = {
 	.name = "error",
-	.version = {1, 3, 0},
+	.version = {1, 4, 0},
+	.features = DM_TARGET_WILDCARD,
 	.ctr  = io_err_ctr,
 	.dtr  = io_err_dtr,
 	.map  = io_err_map,

drivers/md/dm.h

···
 int dm_table_any_congested(struct dm_table *t, int bdi_bits);
 unsigned dm_table_get_type(struct dm_table *t);
 struct target_type *dm_table_get_immutable_target_type(struct dm_table *t);
+struct dm_target *dm_table_get_wildcard_target(struct dm_table *t);
 bool dm_table_request_based(struct dm_table *t);
 bool dm_table_mq_request_based(struct dm_table *t);
 void dm_table_free_md_mempools(struct dm_table *t);

include/linux/device-mapper.h

···
 #define dm_target_is_immutable(type)	((type)->features & DM_TARGET_IMMUTABLE)
 
 /*
+ * Indicates that a target may replace any target; even immutable targets.
+ * .map, .map_rq, .clone_and_map_rq and .release_clone_rq are all defined.
+ */
+#define DM_TARGET_WILDCARD		0x00000008
+#define dm_target_is_wildcard(type)	((type)->features & DM_TARGET_WILDCARD)
+
+/*
  * Some targets need to be sent the same WRITE bio severals times so
  * that they can send copies of it to different devices.  This function
  * examines any supplied bio and returns the number of copies of it the